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        2023
      </a></span></div></div> <div data-v-7dd95ae2><div data-v-7dd95ae2><main class="page" style="padding-right:0;"><section style="display:;"><div class="page-title"><h1 class="title">07 高度塌陷与BFC</h1> <div data-v-8a445198><i class="iconfont reco-account" data-v-8a445198><span data-v-8a445198>hasak</span></i> <i class="iconfont reco-date" data-v-8a445198><span data-v-8a445198>6/18/2022</span></i> <!----> <i class="tags iconfont reco-tag" data-v-8a445198><span class="tag-item" data-v-8a445198>CSS</span></i></div></div> <div class="theme-reco-content content__default"><h1 id="_07-高度塌陷与bfc"><a href="#_07-高度塌陷与bfc" class="header-anchor">#</a> 07 高度塌陷与BFC</h1> <h2 id="_1-高度塌陷"><a href="#_1-高度塌陷" class="header-anchor">#</a> 1.高度塌陷</h2> <p>在浮动布局中，父元素的高度默认是被子元素撑开的</p> <ul><li>当子元素浮动后，会完全脱离文档流，将会无法撑起父元素高度，导致父元素的高度丢失</li> <li>父元素高度丢失后，其下的元素会上移</li></ul> <p>所以高度塌陷是浮动布局中比较常见的一个问题，这个问题我们必须要进行处理</p> <p><img src="https://hasakwebblogimg.oss-cn-hangzhou.aliyuncs.com/202211131002222.gif" alt="高度塌陷"></p> <h2 id="_2-bfc"><a href="#_2-bfc" class="header-anchor">#</a> 2. BFC</h2> <p>**BFC（Block Formatting Context)**块级格式化环境</p> <ul><li>BFC是一个CSS中的隐含属性，可以为一个元素开启BFC</li> <li>开启BFC该元素会变成一个独立的布局区域</li></ul> <p><strong>元素开启BFC后的特点</strong></p> <ul><li>==<strong>不会被浮动元素覆盖</strong>==</li> <li>==<strong>可以包含浮动的元素</strong>==</li> <li><strong>==父子元素外边距不会重叠==</strong></li></ul> <p><strong>BFC的开启方式</strong></p> <table><thead><tr><th style="text-align:center;">元素或属性</th> <th style="text-align:center;">说明</th></tr></thead> <tbody><tr><td style="text-align:center;"><code>&lt;html&gt;</code></td> <td style="text-align:center;">文档根元素</td></tr> <tr><td style="text-align:center;"><strong><code>float:left</code></strong><br><strong><code>float:right</code></strong><br></td> <td style="text-align:center;">浮动元素（flaot不为none）</td></tr> <tr><td style="text-align:center;"><code>position:absolut</code><br><code>position:fixed</code><br></td> <td style="text-align:center;">绝对定位元素</td></tr> <tr><td style="text-align:center;"><strong><code>overflow:hidden</code></strong> <br><strong><code>overflow:scroll</code></strong><br><strong><code>overflow:auto</code></strong></td> <td style="text-align:center;">overflow不为visible和clip的块元素</td></tr> <tr><td style="text-align:center;"><strong><code>display:iniline-block</code></strong></td> <td style="text-align:center;">行内块元素</td></tr> <tr><td style="text-align:center;"><code>display:table-cell</code></td> <td style="text-align:center;">表格单元，默认值</td></tr> <tr><td style="text-align:center;"><code>display:table-caption</code></td> <td style="text-align:center;">表格标题，默认值</td></tr> <tr><td style="text-align:center;"><strong><code>display:table</code></strong><br><code>display:table_row</code><br><code>display:table-row-group</code><br><code>display:tabel-header-group</code><br><code>display:table-footer-group</code><br><code>display:inline-table</code></td> <td style="text-align:center;">匿名的表格单元、分别为HTML表格、表行、<br>表体、表头和表脚的默认值</td></tr> <tr><td style="text-align:center;"><code>display:flow-root</code></td> <td style="text-align:center;"></td></tr> <tr><td style="text-align:center;"><code>display:flex</code><br><code>display:inline-flex</code>的直接子元素</td> <td style="text-align:center;">Flex项，如果他们本身既不是flex，也不是grid或table容器</td></tr> <tr><td style="text-align:center;"><code>display:grid</code><br><code>display:inline-grid</code>的直接子元素</td> <td style="text-align:center;">Grid项，如果他们本身既不是flex，也不是grid或table容器</td></tr> <tr><td style="text-align:center;"><code>contain:layout</code><br><code>contain:content</code><br><code>contain:paint</code></td> <td style="text-align:center;"></td></tr> <tr><td style="text-align:center;"><code>column-count</code>不为auto<br><code>column-width</code>不为auto</td> <td style="text-align:center;">Multicol容器，包含column-count：1</td></tr> <tr><td style="text-align:center;"><code>column-span:all</code></td> <td style="text-align:center;">总是创建一个新的格式化上下文，即column-span：all元素不在multicol容器中</td></tr></tbody></table> <p>但是这些方式都会有一些缺陷。</p> <p><strong>通过其他特殊方式开启元素的BFC</strong></p> <ul><li><strong>设置为浮动（不推荐）</strong>：宽度变化，不再独占一行，下方元素被覆盖。总不能让所有的元素都浮动。</li></ul> <p><img src="https://hasakwebblogimg.oss-cn-hangzhou.aliyuncs.com/202211131045981.gif" alt="float6"></p> <ul><li><strong>设置为行内元素（不推荐）</strong>：宽度变化，不再独占一行，同时与下方元素产生一些空隙</li></ul> <p><img src="https://hasakwebblogimg.oss-cn-hangzhou.aliyuncs.com/202211131047860.gif" alt="float7"></p> <ul><li>设置<code>overflow</code>为非<code>visible</code>值：保持了独占一方的特性（保持了宽度），与下方元素位置没变。
<ul><li>常用的方式为元素设置<code>overflow:hidden</code>（<code>overflow:auto</code>也是可以的）开启其BFC，使其可以包含浮动元素</li> <li><code>overflow:scroll</code>，会出现滚动条，如果不需要滚动条则不推荐</li> <li><code>overflow</code>的方式也会出现一些问题，如不能完全清除div2布局上收到的影响</li></ul></li></ul> <p><img src="https://hasakwebblogimg.oss-cn-hangzhou.aliyuncs.com/202211131050695.gif" alt="float8"></p> <p><strong>总结：</strong></p> <ul><li>可以通过变成浮动元素，来防止自身被浮动元素覆盖</li> <li>可以设置行内块，来防止自身以及其他元素被浮动元素覆盖</li> <li>可以设置<code>overflow</code>属性，包含浮动元素</li></ul> <h2 id="_3-clear"><a href="#_3-clear" class="header-anchor">#</a> 3. clear</h2> <p><code>clear</code>作用：清除浮动元素对当前元素所产生的影响</p> <ul><li><strong>本质是为元素添加一个<code>margin-top</code>属性</strong>， 值由浏览器自动计算，下例添加了200px；</li></ul> <p><strong>可选值：</strong></p> <ul><li><code>left</code>清除左侧浮动元素对当前元素的影响</li> <li><code>right</code> 清除右侧浮动元素对当前元素的影响</li> <li><code>both</code>清除两侧浮动元素中影响较大一侧的元素怒对当前元素的影响</li></ul> <div class="language-CSS line-numbers-mode"><pre class="language-css"><code><span class="token selector">div</span> <span class="token punctuation">{</span>
    <span class="token property">font-size</span><span class="token punctuation">:</span>50px<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.b1</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span>200px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span>200px<span class="token punctuation">;</span>
    <span class="token property">background-color</span><span class="token punctuation">:</span>#bfa<span class="token punctuation">;</span>
    <span class="token property">float</span><span class="token punctuation">:</span>left<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.b2</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span>200px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span>200px<span class="token punctuation">;</span>
    <span class="token property">background-color</span><span class="token punctuation">:</span>orange<span class="token punctuation">;</span>
    <span class="token property">clear</span><span class="token punctuation">:</span>both<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
&lt;div class=<span class="token string">&quot;b1&quot;</span>&gt;1&lt;/div&gt;
&lt;div class=<span class="token string">&quot;b2&quot;</span>&gt;3&lt;/div&gt;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div><p><img src="https://hasakwebblogimg.oss-cn-hangzhou.aliyuncs.com/202211131056118.png" alt="clear1"></p> <h2 id="_4-after"><a href="#_4-after" class="header-anchor">#</a> 4. after</h2> <p>除了以上解决高度塌陷问题的方式，还有<strong>伪元素选择器<code>::after</code></strong></p> <ul><li><code>::after</code>伪元素是一个行内元素，需要转化为块元素，不然撑不起父元素高度。</li></ul> <div class="language-CSS line-numbers-mode"><pre class="language-css"><code><span class="token selector">.box1</span> <span class="token punctuation">{</span>
    <span class="token property">border</span><span class="token punctuation">:</span>10px solid red<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.box1::after</span><span class="token punctuation">{</span>
    <span class="token property">display</span><span class="token punctuation">:</span>block<span class="token punctuation">;</span>
    <span class="token property">content</span><span class="token punctuation">:</span><span class="token string">' '</span><span class="token punctuation">;</span>
    <span class="token property">clear</span><span class="token punctuation">:</span>both<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.box2</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
    <span class="token property">background-color</span><span class="token punctuation">:</span>#bfa<span class="token punctuation">;</span>
    <span class="token property">float</span><span class="token punctuation">:</span>left<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

&lt;div class=<span class="token string">&quot;box1&quot;</span>&gt;
	&lt;div class=<span class="token string">&quot;box2&quot;</span>&gt;&lt;/div&gt;
&lt;/div&gt;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br></div></div><h2 id="_5-clearfix"><a href="#_5-clearfix" class="header-anchor">#</a> 5. clearfix</h2> <p>垂直布局中，会出现边距重叠问题：相邻的垂直方向外边距会发生重叠现象</p> <ul><li>子元素设置了一个<code>margin-top</code>之后，父元素会跟随子元素一起移动，此时margin相邻就会发生重叠</li></ul> <p>可以通过<strong>伪元素</strong>解决，在子父元素之间我们使用<code>before</code>伪元素选择器，将两元素隔开</p> <ul><li><code>content</code>必须得有内容，或者添加<code>display:inline-block</code>属性，这两种方式都会增加一个高度</li> <li>使用<code>display:table</code>可以解决问题
<ul><li>**<code>display:table</code>**也能开启BFC,开启BFC的一个特点就是父子元素外边距不会重叠</li></ul></li></ul> <div class="language-CSS line-numbers-mode"><pre class="language-css"><code><span class="token selector">.box1</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span>200px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span>200px<span class="token punctuation">;</span>
    <span class="token property">background-color</span><span class="token punctuation">:</span>#bfa<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.box1::before</span><span class="token punctuation">{</span>
    <span class="token property">content</span><span class="token punctuation">:</span><span class="token string">' '</span><span class="token punctuation">;</span>
    <span class="token property">display</span><span class="token punctuation">:</span>table<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.box2</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
    <span class="token property">background-color</span><span class="token punctuation">:</span>orange<span class="token punctuation">;</span>
    <span class="token property">margin-top</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

&lt;div class=<span class="token string">&quot;box1&quot;</span>&gt;
	&lt;div class=<span class="token string">&quot;box2&quot;</span>&gt;&lt;/div&gt;
&lt;/div&gt;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div><h3 id="总结"><a href="#总结" class="header-anchor">#</a> 总结：</h3> <ul><li>高度塌陷问题，一般用<code>::after</code></li> <li>外边距重叠问题，一般用<code>::before</code></li></ul> <p>既解决高度塌陷问题，又解决外边距重叠问题： <strong><code>clearfix</code></strong></p> <ul><li><code>.clearfix::before</code> 可以解决外边距重叠问题</li></ul> <div class="language-CSS line-numbers-mode"><pre class="language-css"><code><span class="token selector">.clearfix::before</span><span class="token punctuation">{</span>
    <span class="token property">content</span><span class="token punctuation">:</span> <span class="token string">''</span><span class="token punctuation">;</span>
    <span class="token property">display</span><span class="token punctuation">:</span>table<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><ul><li><code>.clearfix:: after</code>可以解决高度塌陷问题</li></ul> <div class="language-CSS line-numbers-mode"><pre class="language-css"><code><span class="token selector">.clearfix::after</span><span class="token punctuation">{</span>
    <span class="token property">content</span><span class="token punctuation">:</span> <span class="token string">''</span><span class="token punctuation">;</span>
    <span class="token property">display</span><span class="token punctuation">:</span>table<span class="token punctuation">;</span>
    <span class="token property">clear</span><span class="token punctuation">:</span>both<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>父元素引用此类即可</p> <div class="language-CSS line-numbers-mode"><pre class="language-css"><code><span class="token selector">.box1</span> <span class="token punctuation">{</span>
    <span class="token property">border</span><span class="token punctuation">:</span>10px solid red<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.clearfix::before,
.clearfix::after</span> <span class="token punctuation">{</span>
    <span class="token property">content</span><span class="token punctuation">:</span><span class="token string">''</span><span class="token punctuation">;</span>
    <span class="token property">display</span><span class="token punctuation">:</span>table<span class="token punctuation">;</span>
    <span class="token property">clear</span><span class="token punctuation">:</span>both<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token selector">.box2</span> <span class="token punctuation">{</span>
    <span class="token property">width</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
    <span class="token property">height</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
    <span class="token property">background-color</span><span class="token punctuation">:</span>#bfa<span class="token punctuation">;</span>
    <span class="token property">margin-top</span><span class="token punctuation">:</span>100px<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

&lt;div class=<span class="token string">&quot;box1 clearfix&quot;</span>&gt;
	&lt;div class=<span class="token string">&quot;box2&quot;</span>&gt;&lt;/div&gt;
&lt;/div&gt;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div></div></section> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev"><a href="/blogs/基础/html/20221113html06.html" class="prev">
          06 浮动
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